WHOLE-ROCK GEOCHEMISTRY AND PETROGRAPHY OF THE MORNE AUX DIABLES VOLCANIC CENTER, DOMINICA

The island of Dominica, located in the Lesser Antilles Arc in the Caribbean, has one of the highest concentrations of potentially active volcanoes in the world. It is dominated by lava domes, ignimbrites, and block and ash flows that are <100 ka, but recent seismic swarms have occurred beneath the northernmost vent, Morne aux Diables (MAD) which is > 1 Ma. This study characterizes the geochemistry, petrography, and mineral assemblages/textures of pumice, block and ash flows, and lava domes associated with Morne aux Diables. The samples range from 57-64 wt% SiO₂, with pumices slightly less evolved (up to 61 wt% SiO₂). The trace elements are similar for all samples, enriched in LILE and depleted in HFSE. The REE plots display parallel patterns with a slight enrichment in LREE compared to HREE and a pronounced negative Eu anomaly. Two of the lava domes are enriched in all REE in comparison to the other samples. All samples display slight concavity in HREE. The MAD samples are similar to the younger units on the island with respect to major and trace elements, but have a distinctive REE signature, evident on Dy/Dy* vs. Dy/Yb plot. They are slightly more LREE depleted and are less affected by the fractionation of amphibole and/or cpx. The MAD mineral assemblage is plagioclase + opx + Fe-Ti oxides + cpx + hornblende, with rare xenocrysts of quartz and olivine. Many of the phenocrysts (mostly plagioclase and pyroxene) have melt inclusions either randomly dispersed throughout the crystal, in the core, or in thin bands parallel to the crystal boundary. Both plagioclase and pyroxene display disequilibrium textures, with >10 um rims mantling many phenocrysts. In the pyroxenes, the rims were often composed of pyroxenes, clays, and oxides, and were larger surrounding the opx than cpx. Unique to one lava dome sample are opx cores surrounded by thick cpx rims (5-10% of pyx phenocrysts). Pyroxenes are also present in glomerocrysts and feature oxide exsolution textures within glomerocrysts. Collectively, these disequilibrium textures and presence of xenocrysts suggest magma mingling prior to the eruption. Ongoing studies of pyroxene and plagioclase rim and core compositions will allow us to determine if the trigger for the eruption was a mafic injection. This may also help us determine if the eruption dynamics in northern Dominica are unique.